[Form] Add support for Bootstrap 4 forms

[javiereguiluz]

Bootstrap 4 is still in alpha but we should prepare for when it's released (no specific date yet). Regarding the form component, changes from Bootstrap 3 are small, but nonetheless important.

Source: http://v4-alpha.getbootstrap.com/migration/#forms

[FractalizeR]

I think this is incomplete. I tried this, but it looks like form errors are no longer printed in red for example. It can be achieved using text-danger style for example.

[raress96]

Just noticed this and in my opinion for this to be complete it should also include a layout where the custom Bootstrap 4 elements are used.

[szymach]

But Bootstrap4 is still in alpha stage, is there a point introducing a template that may simply stop working after another alpha release?

[hiddewie]

There will be a beta release very soon, see https://github.com/twbs/bootstrap/projects/3.

[szymach]

Well that still may change to various reasons, it would not be uncommon for release date to be postponed. Anyhow, I think it would be better to wait until it starts approaching a stable release, since we have no idea how much it could change.

[PhilETaylor]

Bump - cough - Currently v4.0.0-beta http://getbootstrap.com/

[hiddewie]

@PhilETaylor See PR #21751. It has been open for quite some time now. Any reactions/improvements are welcome. I hope the PR can be merged soon so any Symfony + Bootstrap 4 projects start experimenting with the beta including some nice automatically styled forms :).

[ReSpawN]

We're currently developing a narrowcasting solution with Symfony 3.3.6 and Boostrap 4 beta, and yes, the form styles are currently completely messed up due to all the changes. I'm eagerly looking forward to stable support. Supporting a beta however normally isn't something I would do, but Bootstrap has proven time and time again to be one of the most stable of frontend framework solutions, mainly taking CSS.
How far along is your PR @hiddewie ? I might implement yours before week end.

[hiddewie]

@ReSpawN Thank you for the interest.

The PR should be very functional for the BS 4 beta. I am using it in small personal projects myself, but I have not seen it implemented in large scale projects. The only thing that I was able to determine which does not work is a form widget with invalid state and invalid text for (sets of) radio buttons and checkboxes. I've raised this issuse with the Bootstrap team (twbs/bootstrap#23426).